Our invention relates to prosthetic heart valves, and in particular to prosthetic heart valves which are combined with an integral vascular graft for use in replacing a diseased aortic valve and a portion of the aorta of a patient.
Prosthetic heart valves replace diseased valves in a human heart. These valves fall generally into two categories. Biologic valves are comprised of leaflets made of a flexible biologic material. Depending on the source of the leaflet material, the valve may be either a xenograft, that is, harvested from a non-human cadaver, or an allograft, that is, harvested from a human cadaver. The second major category of prosthetic heart valves is mechanical valves. These valves usually comprise an annular body supporting one, two or three leaflets of a non-biologic material. The annular body and leaflets are frequently formed in pyrolytic carbon, a particularly hard and wear resistant form of carbon. The annular body is captured within a sewing ring so that the valve may be attached to heart tissue at the location of the replaced valve. Mechanical valves with flexible, polymeric leaflets are also known.
Functioning valves are critical to the proper action of the heart. If a valve becomes diseased, it may be replaced by a prosthetic valve. If degeneration of a valve has occurred, however, it is likely that surrounding blood vessels are also diseased. Particularly in the case of the aortic valve, surgeons have found that not only the valve but also the adjacent aorta degenerate. Consequently, both valve and a segment of the ascending aorta may be replaced at the same time. In 1968 Bentall and DeBono described a method for attaching a commercially available graft to a Starr-Edwards mechanical heart valve for the complete replacement of an aneurysmal aorta and aortic valve. See, "A Technique for Complete Replacement of the Ascending Aorta", Thorax, 1968; V. 23, pgs. 338-339. After implanting the mechanical heart valve, a surgeon would stitch a segment of vascular graft to the sewing ring of the mechanical valve. The juncture between the valve and the graft was abrupt and there was usually a sharp change of diameter to be expected between the valve and the graft.
Subsequently, Shiley Corp., in conjunction with cardiovascular surgeons, produced a composite valve and pre-attached graft. Between the valve and the graft, there was a relatively long, tapered fabric section. It was suggested that the taper would provide a smooth transition between the valve and the graft to reduce turbulent flow. Tapered sections of 8 to 12 millimeters have been widely used by Shiley and others offering composite valve/graft combinations.
Combined mechanical heart valves and vascular grafts having a shortened transition area between the valve and the graft are also known. One such combination is disclosed in U.S. Pat. No. 5,123,919. The mechanical valve comprises a rigid circular annular body supporting internal leaflets, a stiffening ring surrounding the annular body, and a sewing ring for attaching the valve to the heart. The stiffening ring also captures a proximal end of the vascular graft between the stiffening ring and the annular body.
From 1968 until 1991, clean, sterile polyester grafts were generally used for combined mechanical heart valves and grafts. In some cases, extremely low porosity grafts were used to minimize leakage, which was reduced to about 50 to 100 cc/min/cm.sup.2. These low porosity grafts were initially successful in minimizing patient blood loss, but they suffered from a long-term failure mode. In use, a neo-intimal layer can build up on the interior surface of the graft where it is constantly exposed to blood flow. Eventually, the neo-intimal layer may become so thick that the shear stress from the flowing blood may peel it off the inside of the smooth, low porosity graft, resulting in a large solid embolism that could cause significant injury to the patient. To avoid this problem, higher porosity grafts, with leak rates in excess of 200 cc/min/cm.sup.2, were employed. These higher porosity grafts allow the neo-intimal layer to grow into the interstices of the graft, creating a mechanical bond that has successfully decreased the problem of neo-intimal layer peeling. Blood loss with higher porosity grafts was minimized by pre-clofting the graft prior to implantation. Typically, the graft was dipped into the patient's own blood. Then the whole assembly was heated for several minutes to dry the blood, producing a semi-impervious coating. Even with this pre-clotting operation, significant blood loss was common. In an effort to decrease this blood loss, the remnant of aortic tissue was frequently wrapped around the graft. This resulted, however, in the formation of a hematoma, capturing blood between the outside of the graft and the inside of the aortic remnant. Professor Cabrol developed a technique of attaching a small (8-10 mm) graft from the site of this hematoma to the right atrium to relieve the interior pressure before the hematoma burst with fatal consequences. Unfortunately, this small graft frequently occluded, so this practice fell out of favor as sealed grafts and fibrin glue came into use.
A pre-clotting procedure adds about 30 minutes to an operation, during which time the patient is subjected to artificial circulation and its associated risks. In some cases surgeons would pre-clot the graft with fibrin glue to avoid this delay. This glue is expensive and has not been widely available in highly regulated jurisdictions because of the risk of disease transmission associated with this human blood product. In 1991, combined mechanical heart valves and vascular grafts became available with pre-sealed, medium porosity grafts. These devices employed collagen or gelatine sealed grafts. This sealing technology prevented significant blood loss through the graft at the time of surgery. After blood flow is re-established, the sealing material dissolves or is digested, leaving a graft with sufficient porosity to eliminate neo-intimal peel.
Aortic heart valves alone are usually implanted within walls of the cardiovascular system at the aortic annulus. Any blood leaking around the heart valve, for example, either past or through the sewing ring of the valve, would nevertheless still be inside the cardiovascular system. Blood would not be leaking into the body cavity as the left ventricle forces blood into the body. Vascular grafts, since they replace a portion of the aorta, have been designed to resist a pressure difference and prevent blood from leaking through the graft into the body cavity. Aortic heart valve and vascular graft combinations, however, have not adequately addressed the problem of blood leakage through and around the sewing ring of the heart valve.
It is a principle object of our invention, therefore, to provide a cardiac valve and vascular graft combination which resists blood leakage around or through the sewing ring of the heart valve.
Another object of our invention is to provide a valve/graft combination with a relatively impervious sewing ring.
Another object of our invention is to provide a sewing ring in a heart valve/graft combination with a solid silicone baffle.
These and other objects and features of our invention will be apparent from the following description taken with reference to the accompanying drawings.